ISIS and al-Qaida linked group the Nusra Front are exempted from the month-long cessation of war.

Before the ceasefire, Syrian rebels fighting president Bashar al-Assad's regime had also been targeted by Russian warplanes, western countries alleged.

Air strikes in Syria leave waves of destruction

Air strikes in Syria leave behind devastating consequences in the war against Daesh militants.

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AFP/Getty Images

A man stands on the rubble of a destroyed building following reported air strikes by government forces in the rebel-held Shaar neighbourhood of the northern city Aleppo

Assad's troops and the moderate rebels have laid down their weapons as part of the truce.

Syrian government forces have been able to advance into Palmyra helped by support from Russian airstrikes in a major assault against ISIS.

Waves of explosions were spotted in Palmyra as tanks and armoured vehicles fired from the outskirts.

The Syrian Observatory for Human Rights said it was the biggest assault in a three-week campaign by the Syrian army and allied militia fighters to recapture the desert city and open up the road to ISIS strongholds further east.

Troops have advanced into the ancient city and taken several neighbourhoods.

The recapture of Palmyra, which ISIS seized in May 2015, would mark the biggest reversal for them in Syria since Russia's intervention turned the tide of the five-year conflict in President Bashar al-Assad's favour.

The limited ceasefire has allowed aid to reach previously inaccessible areas under siege.

Peace talks in Geneva are ongoing but progress has been slow, with the government and opponents disagreeing over whether Assad must leave power.